Though accumulation of solids on the surface of a membrane diminishes permeate flux ("flux", volume of permeate per unit area of membrane per day) in all types of membranes, the severity of the problem is diminished in applications in which water to be filtered is flowed so that it contacts the membrane tangentially. In tangential flow, water to be filtered (referred to as "dirty" water) flows across and along the outer surface of the membrane in well-defined channels, as does the retentate, and the permeate flows through the membrane in a direction orthogonal to the flow of dirty water and retentate. Dislodging solids carried onto the membrane is assisted by this tangential flow as long as flow of dirty water is continuous and relatively fast. Tangential flow occurs with the use of a flat sheet membrane or assembly of flat sheets (such as is shown in U.S. Pat. No. 5,192,456 to Ishida et al., U.S. Pat. No. 4,735,718 to Peteres and U.S. Pat. No. 5,460,720 to Schneider); or with use of a tube type membrane wherein a large diameter tube has a membrane coated on its inner surface, such as is shown in U.S. Pat. No. 4,897,191 to Langerak et al. Transverse flow occurs when filtering with a cartridge of hybrid frameless arrays of fibers or in a module containing an assembly of cartridges, as shown in U.S. Pat. No. 5,182,019 to Cote et al. In general, one expects to encounter less fouling due to build-up of solids on fiber surfaces when the dirty water is flowing transversely to the direction in which the fibers lie, as compared to fouling in tangential flow. However devices are used in which fibers are confined in a shell to provide a well-defined tangential flow, despite the relatively low membrane-packing density in such devices; and such devices consume far more energy than an unconfined vertical skein of fibers.
Vertical skeins of fibers used for "outside-in" filtration without being confined in a shell, offer high membrane-packing density but do not lend themselves to having their surfaces subjected to well-defined tangential flow of dirty water. An unconfined bundle or skein of fibers in a reservoir of dirty water does not provide tangential flow of dirty water and retentate unless aerated so that liquid flows upwards with rising bubbles. However, the velocity of flow of dirty water in tangential flow, parallel to the surface of the membrane, is relatively low unless the skein is subjected to a column of fast-rising air bubbles.
One solution to alleviate the problem of solids accumulation on the surfaces of hollow fiber membranes is periodically backflushing with permeate. This solution is suitable for low concentrations of solids, particularly those solids which do not have a high affinity for the membrane's surface, referred to as "non-sticky" solids. Such non-sticky solids, for example, are inorganic particles of bentonite, silica, talc and organic agglomerates which are not highly hydrophilic. However, even with such non-sticky solids, the frequency of backflushing increases in proportion with the concentrations of solids, so that the amount of water used for backflushing makes the process less economically attractive.
Another solution to the problem of accumulating solids on the surfaces of hollow membranes comprises backblowing air under pressure through the membrane and periodically flushing the outer surfaces of the membranes with a stream of high velocity water as disclosed in patents assigned to Memtec U.S. Pat. Nos. 5,643,455 and 4,935,143 to Kopp et al. This solution of backblowing air is suitable for skeins of hollow fibers held in a restrictive containment vessel and the dirty water contains a relatively low concentration of solids.
Yet another solution consists in scrubbing the solids from the surfaces of slack fibers between fixed upper and lower headers of a vertical skein with a continuous flow of coarse air bubbles as disclosed in U.S. Pat. Nos. 5,639,373 and 5,783,083 to Mahendran et al. Scrubbing a stationary skein of fibers with a column of rising air bubbles, is economical when it is practiced on an assembly of vertical skeins, but the skein is required to have slack fibers critically spaced-apart so that individual fibers are circumferentially sealed by potting resin in opposed headers. The manner in which the fibers are potted so as to be individually sealed in a film of surrounding resin is not critical so long as the fibers are spaced apart to ensure an essentially leak-proof header. Such an assembly of vertical skeins is referred to herein as a "cassette" and it is known that a cassette is an expedient structure for the deployment of vertical skeins, generally. In a cassette, multiple skeins from 2 to about 20 or more, preferably from about 4 to 10, are secured in a framework. The framework may be rectangular and may have vertical skeins with either cylindrical or rectangular parallelpiped headers secured in the framework. For maximum density, cylindrical skeins may be secured in a generally cylindrical framework. For simplicity and convenience, the description herebelow is specifically directed to a cassette in which from 6 to 8 skeins with rectangular parallelpiped headers, each skein comprising multiple, generally planar arrays of hollow fibers, spaced apart in the headers by flexible spacing means, and having a specified amount of slack. The skeins are assembled side-by-side, sequentially, along the longitudinal axis (x-axis) of the cassette, the arrays of each skein lying in the transverse direction (y-axis) in a vertical plane.
From the foregoing, and knowing that translating a single vertical skein to-and-fro, into and out of a vertical column of air bubbles generated by a stationary diffuser necessarily diminishes the exposure of fibers in the skein to the scrubbing action of the bubbles, it was surprising to find that using a cassette and oscillating it with specified limited displacement at specified frequencies, imparts the requisite motion which minimizes the build-up of solids within each skein of the cassette, and between adjacent skeins thereof. Hence, the particular solution disclosed herein to the problem of solids build-up, is particularly adapted to a shell-less cassette of skeins of hollow fibers immersed in liquid to be filtered.
Solutions to prevent solids build-up or to facilitate their removal from a module of hollow fibers, have been disclosed in the following Japanese patent applications which teach agitating the module:
JPA H9-075687 filed on Sep. 14, 1995 by Kurita Water discloses a module formed from hollow fibers woven as a screen placed horizontally in a tank and air bubbles rise from under the screen. The screen is raised and lowered to obtain uniformity of distribution of air bubbles and purports to change the direction of air bubbles with the membranes. Raising and lowering the screen with fibers in the horizontal plane, generates a transverse flow of dirty liquid being filtered relative to the surfaces of the membranes being moved vertically.
JPA H9-206565 filed on Feb. 2, 1996 by Kurita Water teaches a screen type module of hollow fibers subjected to air bubbles rising from beneath the module. The screen type module includes opposed frames, one of which is vertically movable relative to the other. During filtration, the vertically reciprocable frame is moved up and down while the other frame is stationary. As in the prior Japanese disclosure, relative tangential flow is generated by the vertically moving screen type module.
In each of the foregoing disclosures a single module is used, and it is translated through the water to be filtered while the module is confined in a tank in such a manner as to effectively provide transverse flow of water across the length of the hollow fibers. As stated in each disclosure, enhancement of filtration efficiency is evident in a single module because of the effectively transverse flow. In contrast, no improvement in filtration is evidenced in a single vertical skein because the flow of water, dictated by the velocity of the bubbles rising vertically, is essentially parallel to the surfaces of the vertical fibers and is not in transverse flow.
JPA H8-332357 filed on Jun. 6, 1995 by Toray teaches cleaning a module of hollow fiber membranes by vibrating the module while back-washing it. Vibrating the module varies the water level and helps remove solids dislodged during back-washing thus cleaning the surfaces of the fibers. The vertical configuration of fibers in a module is maintained during its reciprocation while backwashing; this causes movement of the water to clean the surfaces of the fibers. Improvement in efficiency is attributed to each module being confined in a shell.
The present disclosure is specifically directed at improving the effectiveness of an assembly of multiple vertical hollow fiber skeins (referred to as a "cassette") which, during operation, is immersed in a relatively large and unrestrictive reservoir. Such a cassette is referred to herein as "shell-less" because it is not confined in a shell which directs flow of fluid to be filtered in one direction. In a cassette, the flow of permeate is essentially parallel to the bubble-generated flow of substrate to be filtered, and the fibers are subjected to essentially tangential flow during filtration. When oscillating, that portion of the cassette on each side of its vertical central axis is aerated continuously, such aeration being switched from one side to other.